9 Tube Stereo Amplifier

20 Watts per channel Maximum


Want big tube room filling sound without the trouble or expense of big tube equipment? Look no further! This 9 tube amp is simple to operate, powerful, and uses only 3 types of tubes. This amp features a 6BQ5 push-pull output for each channel, and two quiet, reliable 6EU7 tubes in each channel as well. A 5U4-G keeps plenty of power flowing. You have complete control with powerful bass and treble tone controls, and a balance control compensates for any speaker/room or source audio deficiencies.

Schematic:

The Magnavox-style output section uses a paraphase inverter and a minimum of circuit components. An inverse feedback loop minimizes distortion to give you clean, room filling tube sound. This is a powerful amplifier, if you live in a dorm room or small apartment, you may have difficulty, as this amplifier does not like to play at low volume levels.

Parts List

Control Resistors
Resistor ID Value (ohms) Function, Notes
R1A & B Dual-section 500K audio taper 1/2W Volume Control
R2A & B Dual-section 1M linear taper 1/2W Bass Control
R3A & B Dual-section 1M linear taper 1/2W Treble Control
R4 750 linear taper 1W Balance Control
Fixed Resistors
R5, R23 2.2K 1/2W Input amp cathode
R6, R24 220K 1/2W Input plate load
R7, R25 10K 1/2W Bass tone comp
R8, R26 100K 1/2W Bass tone comp
R9, R27 100K 1/2W Audio coupling
R10, R28 2.7K 1/2W Tone amp cathode
R11, R29 82K 1/2W Tone amp plate load
R12. R30 470K 1/2W Grid load
R13, R31 47K 1/2W Grid input
R14, R32 5.6K 1/2W Inverter cathode
R15, R33 2.2K 1/2W Driver cathode
R16, R34 220K 1/2W Plate load
R17, R35 220K 1/2W Plate load
R18, R36 220K 1/2W Output grid
R19, R37 39K 1/2W Inverter grid
R20, R38 470K 1/2W Output grid
R21, R39 220K 1/2W Monitor isolation
R22, R40 3.9K 1/2W Feedback coupling
Power Resistors
R41 100 10W wire wound Output cathodes
R42 10K 5W wire wound Voltage divider
R43 15K 2W metal oxide Voltage Divider
R44 750 5W wire wound potentiometer Hum adjust

 

Fixed Capacitors
Capacitor ID Value Function, Notes
C1, C10 0.047mF 630V Audio coupling
C2, C11 0.002mF 630V Bass tone comp
C3, C12 0.02mF 630V Bass tone comp
C4, C13 0.0002mF 630V Treble tone comp
C5, C14 0.002mF 630V Treble tone comp
C6, C15 0.047mF 630V Audio coupling
C7, C16 100mmf 630V B+ decoupling/snubber
C8, C17 0.0047mF 630V Audio coupling
C9, C18 0.047mF 630V Audio coupling
C19 0.01mF 1400V Line bypass
Electrolytic Capacitors
C20 22mF 25V Cathode bypass
C21 47mF 450V Filter
C22 33mF 450V Filter
C23 10mF 450V Filter
C24 33mF 450V Filter

 

Choke ID Inductance (henries @ 1000 cps) Current, notes
L1 1 henry 130 ma @ 330 volts, U-bracket

 

Power Transformer, T1
Primary Sec. 1 Sec. 2 Sec.3
117V AC 60cps .92A 600V CT 130ma 5 volts 3 amps 6.3 volts 4 amps

 

Output Transformers T2, T3
Primary Impedance Secondary Impedance
8000 ohms,CT @ 400 cps Matched to speakers, with a tap @ 4 ohms for feedback.

 

Miscellaneous Parts
Part ID Description Function, Notes
SW1A & B 2-pole, 6-throw make-before break switch Audio Selector Switch
SW2 Single-pole, single-throw switch, 5A rated Power Switch
V1 6EU7 Input amp-tone amp left channel
V2 6EU7 Driver-phase inverter left channel
V3 6EU7 Input amp-tone amp right channel
V4 6EU7 Driver-phase inverter right channel
V5 6BQ5 Output left channel
V6 6BQ5 Output left channel
V7 6BQ5 Output right channel
V8 6BQ5 Output right channel
V9 5U5-G Current rectifier

 


Created 11-21-09

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